I ride the 2007 sumpy frame(XL) style,although even at 6'2" my legs are shorter in relevance to my torso and arm length and actually dont use all the seat rail adjustment towards the back when I plumb bob my knee joint to the axis of my pedals.

Once you get your seat height correct and your knee correctly over the pedal axial then itís just a matter of whether you have a lot of seat rail hanging off the back of the seat clamp or not. If you do have a lot of rail hanging off the back, then get a set back post to reduce the chance of breaking your seat rails. There can be a big difference between saddle manufacturers on the bias for and aft on seat rail placement so the poll is not that useful.

I've had both straight and setback post on my 07 stumpy. I prefer the setback because i like the flexibility of adjusting the seat position (forward and back) especially now that changed to a shorter stem.

What are the advantages of a set back? I've never messed with mine, just ridden it like it was set up and haven't had any problems.

It's not for performance advange, it's to set up the cockpit correctly. Just as steveccnv says above. Most seatposts incorporate some sort of set back at the seat rail clamp in their design, but Thomson does not (the seat rail clamps are directly above the center of the post). Therefore, Thomson sells the set back post to accomodate those that need some setback to get the right cockpit position.